[format.string] Separate out the general (type-independent) description

of formatting from the format specifiers for arithmetic and string
types, and make the presentation of the latter consistent with the
presentation for chrono types.

Author: zygoloid

source/time.tex

@@ -10165,7 +10165,9 @@
 in the chrono library\iref{time.syn}
 meets the \newoldconcept{Formatter} requirements\iref{formatter.requirements}.
 The \tcode{parse} member functions of these formatters
-interpret the format specification according to the following syntax:
+interpret the format specification
+as a \fmtgrammarterm{chrono-format-spec}
+according to the following syntax:
 
 \begin{ncbnf}
 \fmtnontermdef{chrono-format-spec}\br

source/utilities.tex

@@ -19618,6 +19618,8 @@
 
 \rSec2[format.string]{Format string}
 
+\rSec3[format.string.general]{In general}
+
 % FIXME: For now, keep the format grammar productions out of the index, since
 % they conflict with the main grammar.
 % Consider renaming these en masse (to fmt-* ?) to avoid this problem.
@@ -19639,7 +19641,7 @@
 
 \begin{ncbnf}
 \fmtnontermdef{replacement-field}\br
-    \terminal{\{} \opt{arg-id} \opt{format-spec} \terminal{\}}
+    \terminal{\{} \opt{arg-id} \opt{format-specifier} \terminal{\}}
 \end{ncbnf}
 
 \begin{ncbnf}
@@ -19671,13 +19673,23 @@
     \terminal{0 1 2 3 4 5 6 7 8 9}
 \end{ncbnf}
 
+\begin{ncbnf}
+\fmtnontermdef{format-specifier}\br
+    \terminal{:} format-spec
+\end{ncbnf}
+
+\begin{ncbnf}
+\fmtnontermdef{format-spec}\br
+    \textnormal{as specified by the \tcode{formatter} specialization for the argument type}
+\end{ncbnf}
+
 \pnum
 The \fmtgrammarterm{arg-id} field specifies the index of
 % FIXME: "args" hasn't been introduced yet!
 the argument in \tcode{args}
 whose value is to be formatted and inserted into the output
 instead of the replacement field.
-The optional \fmtgrammarterm{format-spec} field
+The optional \fmtgrammarterm{format-specifier} field
 explicitly specifies a format for the replacement value.
 
 \pnum
@@ -19688,7 +19700,9 @@
 \end{example}
 
 \pnum
-If all \fmtgrammarterm{arg-id}s in a format string are omitted,
+If all \fmtgrammarterm{arg-id}s in a format string are omitted
+(including those in the \fmtgrammarterm{format-spec},
+as interpreted by the corresponding \tcode{formatter} specialization),
 argument indexes 0, 1, 2, \ldots{} will automatically be used in that order.
 If some \fmtgrammarterm{arg-id}s are omitted and some are present,
 the string is not a format string.
@@ -19710,27 +19724,44 @@
 \pnum
 The \fmtgrammarterm{format-spec} field contains
 \defnx{format specifications}{format specification!format string}
-that define how the value should be presented,
-including such details as
-field width, alignment, padding, and decimal precision.
+that define how the value should be presented.
 Each type can define its own
-formatting mini-language or
 interpretation of the \fmtgrammarterm{format-spec} field.
-The syntax of format specifications is as follows:
+\begin{example}
+\begin{itemize}
+\item
+For arithmetic, pointer, and string types
+the \fmtgrammarterm{format-spec}
+is interpreted as a \fmtgrammarterm{std-format-spec}
+as described in \iref{format.string.std}.
+\item
+For chrono types
+the \fmtgrammarterm{format-spec}
+is interpreted as a \fmtgrammarterm{chrono-format-spec}
+as described in \iref{time.format}.
+\item
+For user-defined \tcode{formatter} specializations,
+the behavior of the \tcode{parse} member function
+determines how the \fmtgrammarterm{format-spec}
+is interpreted.
+\end{itemize}
+\end{example}
 
-% FIXME: Listing chrono-format-spec here is a mistake. We should have a clean
-% separation between the extensibility mechanism and the extensions to it.
-\begin{ncbnf}
-\fmtnontermdef{format-spec}\br
-    \terminal{:} std-format-spec\br
-    \terminal{:} chrono-format-spec\br
-    \terminal{:} custom-format-spec
-\end{ncbnf}
+\rSec3[format.string.std]{Standard format specifiers}
 
-\begin{ncbnf}
-\fmtnontermdef{custom-format-spec}\br
-    \textnormal{user-defined syntax dependent on the type of the argument}
-\end{ncbnf}
+\pnum
+Each \tcode{formatter} specializations
+described in \ref{format.formatter.spec}
+for fundamental and string types
+interprets \fmtgrammarterm{format-spec} as a
+\fmtgrammarterm{std-format-spec}.
+\begin{note}
+The format specification can be used to specify such details as
+field width, alignment, padding, and decimal precision.
+Some of the formatting options
+are only supported for arithmetic types.
+\end{note}
+The syntax of format specifications is as follows:
 
 \begin{ncbnf}
 \fmtnontermdef{std-format-spec}\br
@@ -19774,19 +19805,6 @@
     \terminal{a A b B c d e E f F g G n o p s x X}
 \end{ncbnf}
 
-where
-\fmtgrammarterm{std-format-spec}
-defines a common formatting mini-language
-supported by fundamental and string types,
-\fmtgrammarterm{chrono-format-spec}
-defines a mini-language for chrono types\iref{time.format}, and
-\fmtgrammarterm{custom-format-spec}
-is a placeholder for user-defined mini-languages.
-\begin{note}
-Some of the formatting options
-are only supported for arithmetic types.
-\end{note}
-
 \pnum
 \begin{note}
 The \fmtgrammarterm{fill} character can be any character
@@ -20514,10 +20532,14 @@
 \item a floating-point type.
 \end{itemize}
 \end{itemize}
-
+The \tcode{parse} member functions of these formatters
+interpret the format specification
+as a \fmtgrammarterm{std-format-spec}
+as described in \ref{format.string.std}.
 \begin{note}
 Specializations such as \tcode{formatter<wchar_t, char>}
-and \tcode{formatter<const char*, wchar_t>} that require implicit
+and \tcode{formatter<const char*, wchar_t>}
+that would require implicit
 multibyte / wide string or character conversion are disabled.
 \end{note}